Hinton (1984) described an unusual mutator in D. ananassae which he speculated was a transposable element, tom, with (previously) novel properties: the element could only be detected phenotypically by its effect on eye morphology at 21 non-pleiotropic non-dosage compensated, non-randon loci. Because of the recovery of a spontaneous singed mutation, sn9g, in an ocullar morphology (Om) mutant line derivative, Om(1D)9g, it was possible to investigate his speculation. D. melanogaster singed, sn, DNA probes were used to isolate and recover a D. ananassae singed gene. A comparison of the sn9g and wild type singed restriction map implicated a 6.5 kb insert as the element responsible for the mutator effect. A total of 186 recombinant lines from four X-linked Om loci were examined. 80 were Om and 106 were non-Om; in all instances an in situ hybridization signal, when probed with sn9g insert, was found at appropriate locations on the polytene chromosomes. Linkage was complete and showed that the sn9g insert was homologous to sequences localized at the sites of Om mutants. Preliminary analysis of several isolated clones of the tom element indicate a fairly conserved structure typical of copia-like elements. DNA sequence analysis of the tom at sn9g and Om(1D)9g showed direct repeats at the termini that is also characteristic of copia-like elements. Ongoing southern blot studies of several Om(1D) mutations indicate that some are due to insertions of a single tom, others appear to be due to multiple copies and yet others show no obvious alterations in the cloned region surrounding the tom insertion site in Om(1)9g. Further investigations of the tom elements and the Om mutations should provide insight into the mechanisms underlying this very unusual mutational process.